LeetCode 3950: Exactly One Consecutive Set Bits Pair

Meta Data

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Difficulty: easy First Attempt: 2026-06-11

• Submissions: Accepted (first try)

Approach

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n >> 1 shifts every bit right by one; ANDing with n, bit i is 1 when bits i and i + 1 of n are both 1 (Bit Manipulation Template 2). So the number of ones in pairs = n & (n >> 1) is the number of adjacent pairs; exactly one means pairs is non-zero with a single 1. Overlapping pairs count, e.g. 111 has two.

My Solution

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class Solution:
    def consecutiveSetBits(self, n: int) -> bool:
        pairs = n & (n >> 1)
        return pairs != 0 and pairs & (pairs - 1) == 0

I originally converted bin(n) to a string and counted "11" position by position, which also counts overlaps.

Complexity

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Time O(1), space O(1).

Mistakes to Avoid

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• str.count("11") skips overlaps ("111".count("11") is 1); compare position by position or use bit operations.

Interview Explanation

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Each 1 in n & (n >> 1) marks one adjacent pair, so I check it is non-zero and that x & (x - 1) is 0, i.e. exactly one 1. Time O(1).